Coating apparatus and coating method for moving webs



Feb. 15, 1966 R. w. FOWELLS ETAL 3,235,401

COATING APPARATUS AND COATING METHOD FOR MOVING WEBS Filed March 11, 1963 INVENTORS ROBERT w FOWELLS h I I]: BY BYRON c. SHAWCROSS AGENT United States Patent 3,235,401 CQATING APPARATUS AND C(PATENG METHOD FOR MOVING WEBS Robert W. lFowells, Syracuse, N.Y., and Byron C. Shawcross, Vancouver, Wash, assignors to Crown Zellerbach Corporation, San Francisco, Calif, a corporation of Nevada Filed Mar. 11, 1963, Ser. No. 264,210 9 Claims. (Cl. 1l.7102) This invention relates to a coating apparatus and a coating method for coating of a continuous flexible Web at high speeds. More particularly this invention relates to an apparatus and a coating method employing a bar means and an air knife for removal of excess of coating material on a moving web at high speeds to obtain a uniform, smooth, and high gloss coating surface.

In the past rods, scraper blades, and rolls have been used in coating operations as mechanical doctor devices to remove excess coating materials from a moving coated web. However, the finished coating surface produced by employing such doctor devices lacks un formity in coating thickness. The use of air knife doctors to remove the excess coating and to smooth the remainder of the coating materials is well-known. While the method of smoothing a coating with air knife doctors is superior to that with the above mentioned mechanical doctor devices, many problems have become evident as high viscosities and fast coating speeds have been attempted. Air knife doctors are, for practical purposes, limited in coating speed because of turbulence in the stream of air leaving the air knife lips. This turbulence causes uneven coatings and may produce a fog with thin coating materials which then drifts to the completed coated web giving it an unsightly and grainy appearance. In addition, higher speeds require higher air pressures which in turn result in foam produced in the blow-01f of many coating materials. Foam is a serious problem particularly where large amounts of coating material are continuously circulated from the blow-off to a coating container from which the coating material is applied onto a moving Web.

Air turbulence problems have been attacked in many ways by attaching bafiles or streamlined surface on or near the air knife lips to control the flow of air. Other air knife systems have been provided with a means to evacuate air from the area of the lips to avoid fogging of the finished coating. Even though these modifications have been improvements in the art, the basic problem of air turbulence has become increasingly evident with increased coating speeds.

In the case of air knife doctors, all excess coating material is removed by the impinging air stream. Viscous materials, however, present a problem for two reasons: First, higher air pressures are needed to doctor viscous coating materials resulting in increased turbulence in the doctored excess material which leads to uneven coatings. This problem is usually resolved by applying the coating materials at lower speeds. Secondly, higher air pressures required to doctor viscous coating materials may cause air bubbles to be entrained in the finished coating.

Accordingly, an object of this invention is to provide an apparatus and a method for coating web materials with a liquid coating material at high speeds.

Another object is to provide a method and an apparatus which employs, in combination, one or more bar means and one or more air knife doctors for uniformly applying a liquid coating material, at low or high viscosities, to a Web moving at low or high speeds and obtaining a uniform, smooth, and high gloss coating surface.

3,235,4M Patented Feb. 15, 1966 Other objects and advantages of this invention will be apparent from the following description, the accompanying drawing, and the appended claims.

The present invention comprises an apparatus and a method for applying a uniform, smooth, and high gloss coating of a desired wet thickness onto a moving web including means for applying onto the web a layer of coating material; a bar means, such as a rod, for removing from the web a desired amount of excess coating material; and means for directing at the surface of the coating a stream of a gas, such as air, compatible with the composition of the coating, thereby removing any remaining excess coating material and obtaining a uniform, smooth, and high gloss coating surface.

A preferred embodiment of this invention will now be specifically described with reference to the accompanying drawing wherein:

FIG. 1 is a front sectional view of the preferred embodiment of the invention.

FIG. 2 is a side View of a bar means assembly.

FIG. 3 is a view showing portions of a wire-Wound rod, such as may be used in a modification of the invention.

Referring now to FIGS. 1 and 2 a web 1, such as a paper web, which may move at a wide range of speeds, such as 1000 feet per minute or higher, travels around a positioning roll 3 which causes the web to contact the coating applicator roll 5. Coating material 7 of a desired composition and viscosity is brought and applied to web 1 by coating applicator roll 5 to produce a coated web. Coating applicator roll 5 may be rotating in either direction with or against the direction of web travel, and at a speed equal to, greater, or slower than the speed of the web. it is preferable, however, to adjust positioning roll 3 so that the area of contact between the web under tension and the roll is more than just the tangent line at the point where web 1 and applicator roll 5 meet. Applicator roll 5 is positioned so that a portion thereof is immersed in coating material 7 in a coating container, such as coating pan 15. The amount of coating material 7 applied onto web 1 is adjusted by the speed of the applicator roll and the web. The method of application of the coating material onto the web is not critical. Any known method for achieving the abovementioned purpose is satisfactory.

Once coating material 7 is applied onto moving web 1 by applicator roll 5 the coated web moves toward a bar means, such as rod 9, whereby any desired amount of the excess coating material may be removed from the coated web. The bar means may be a tear drop, a knife design, a taut wire or preferably a rod which may, if desired, have a smooth, indented, or wire-wound surface. FIG. 3 shows portions of a rod of the latter type, having wire 12 wound around the outer surface of bar 10. Rod 9 is adjustably positioned on its support 13 so that it contacts the surface of the coated web and removes any desired portion of the excess coating material. Any distance between rod 9 and the surface of the uncoated web is governed by the desired wet thickness of the coating or the amount of coating material to be removed. However, it is preferable that rod 9 be adjusted so that it creates a tension against the web. The amount of the tension depends on the coating speed, geometry of the system, and the desired coating thickness. Rod 9 may be positioned anywhere between a point where the coating material is applied onto said web and a point P on the coated web where P is, in relation to the direction of web travel, before the first point of contact of a backing means, such as backing roll 17, with the coated web. However, it is preferable to position rod 9 in such manner and such distance from applicator roll 5 that the removed excess coating material is caused to flow into .3 coating pan where it may again be applied onto web 1. The diameter of rod 9 may vary depending upon viscosity of coating material '7, the speed of coating, and the coating weight.

If a rod is employed for bar means 9 it may, if desired, be a rod which rotates in its channel support 11. Channel support 11 may have indentations therein for the purpose of cleaning out the rod from coating materials which may accumulate among the grooves of the indented or wire-wound rod. The rod may rotate around its axis in either direction at a speed faster or slower than the speed of the web. Drive motor 23 and reducing gear 25 may adjust the speed of the rod in a wide range, such as between 1 to 1400 feet per minute or higher.

After a desired amount of the excess coating material has been removed by rod 9 the coated web is moved on toward a coated web backing means, such as backing roll 17, where a stream of a gas, such as air, compatible with the coating composition is directed from a device, such as an air knife doctor 19, at the surface of the coating at an impingement line transverse to the direction of Web travel. It is preferable to situate the air knife so that the air impingement line is formed on that portion of the coated web which is in contact with the backing roll 17. Air knife doctor 19 is positioned at a certain distance from rod 9 so that the two devices act independently and successively to control coating weight and thickness. Since one of the main objects of this invention is to apply a coating material onto a moving web at high speeds and to prevent entrainment of streaks and coating material bubbles in the finished coating, it is essential to avoid a contact of the main force of the air stream with the excess coating material being removed by rod 9. To achieve this purpose, air knife doctor 19 must be so situated that the air impingement line caused by the air knife is a sufficient distance away from and beyond rod 9 in the direction of web travel. The term main force of the air or gas stream denotes that force of the gas which upon contact with the excess coating material causes sufficient turbulence and air bubbles which may be entrained in the finished coating resulting in an uneven coating surface. The main force of the gas varies depending upon the gas pressure required to doctor the coated web. Accordingly, the distance between rod 9 and the air knife 19 may vary. However, air knife 19 must be positioned a sutficient distance apart from rod 9 so that the air stream from the air knife does not co-act with rod 9 at the place where the excess coating material is being removed by rod 9. In this manner the unevenness of the finished coating surface, which may be the result of'close proximity of air impingement line to rod 9, is avoided. FIG. 1 illustrates a preferred positioning of air knife 19 and its distance from rod 9. The air pressure from air knife doctor 19 is adjusted so that it removes any remaining excess coating material from the web whereon a desired uniform wet coating thickness and a smooth high gloss coating surface are produced. The finished coated surface is characterized by a more uniform coating thickness and a smoother and higher gloss coating surface than that produced by either rod 9 or air knife alone. The finished coated surface is then moved on toward a dryer to be dried by any coventional means. The excess coating material removed by air knife doctor 19 falls into a blowotf container, such as pan 21, which may, if desired, continuously empty its content into coating pan 15.

One of the advantages of the present invention resides in the position of rod 9. Rod 9 may be easily adjusted for any coating thickness and readily positioned anywhere between the point where the coating material is applied onto the web and the point P. The farther the rod is from the air impingement line and the closer it is to the point of application of coating material onto the web the better coating is obtained. The rod is preferably positioned so that the excess coating material removed by the rod fiows into the coating applicator pan 15. Any portion or all of the excess coating material may be removed by rod 9. In most instances, it is advantageous to remove all or a major portion of the excess coating material with rod 9 and to use the air knife doctor 19 for a final coating weight adjustment and for obtaining a more uniform coating thickness and a smoother and higher gloss coating surface. Removal of a small amount of coating material by the air stream results in lower air pressures, less foam, fewer bubbles in the coated surface, and attainment of high coating speeds. If all of the excess coating material is removed by rod 9, the air knife 19 can be used to brush or smooth the coating to obtain greater smoothness and higher gloss. For example, in the case of a highly viscous coating material, removal of all or nearly all of the excess coating material reduces the doctoring load of the air knife 19 allowing higher speeds without danger of entraining air bubbles in the coating surface. Thus, increased flexibility in control of the speed of the web, coating weight, and smoothness and gloss of the coating surface are achieved by the apparatus and method of this invention.

The following examples will illustrate but in no way limit the scope of the present invention:

Example 1 A vinylidene-acrylic latex coating having a 60.2% solids content and a viscosity of 25 centipoises is prepared in a conventional manner. The coating apparatus employed is of the type shown in FIG. 1. A rotating rod having a diameter of 0.375 inch is used. The rod is placed adjacent to the coating applicator roll. The rotation of the rod is in the same direction as the web travel. The coating material is introduced into the coating pan as described above and applied to a moving web of paper at 800 feet per minute with an air knife pressure of 0.6 pound per square inch. 70% of excess coating material is removed by the rod. The remaining 30% excess coating material is removed by the stream of air from the air knife. The amount of foam in the air knife blow-off pan is small and a uniform, smooth and high gloss coating on the paper web is obtained.

Example 2 A vinylidene-acrylic latex coating having a 45% solids content and a viscosity of 25 centipoises is prepared in a conventional manner. The coating apparatus employed is of the type shown in FIG. 1. A rotating wire-wound rod having a rod diameter of 0.375 inch and a wire diameter of 0.014 inch is used. The rod is placed adjacent to the coating applicator roll. The rotation of the wirewound rod is in the opposite direction to the web travel. The coating material is introduced into the coating pan as described above and applied to a moving web of paper at 800 feet per minute with an air knife pressure of 1.0 pound per square inch. 95% of excess coating material is removed by the rod. The remaining 5% excess coating material is removed by the stream of air from the .air knife. The amount of foam in the air knife blow-off pan is nil and a uniform, smooth and high gloss coating on the paper web is obtained.

Example 3 A vinylidene-acrylic latex coating having a 55% solids content and a viscosity of 1000 centipoises is prepared in a conventional manner. The coating apparatus employed is of the type shown in FIG. 1 A rotating rod having a diameter of 1 inch is used. The rod is placed adjacent to the coating applicator roll. The rotation of the rod is in the opposite direction to the web travel. The coating material is introduced into the coating pan and applied to a moving web of paper at 800 feet per minute with an air knife pressure of 1.0 pound per square inch. of excess coating material is removed by the rod. The remaining 20% excess coating material is removed by the streamof air from the air knife. The amount of foam in the air knife blow-off pan is very small and a uniform smooth and high gloss coating on the paper web is obtained.

The apparatus and method of the present invention increase the capabilities of the ordinary air knife by giving higher gloss and smoother coatings at high speeds of the web. Coating quality depends primarily on its smoothness for enhanced appearance and, in the case of protective coatings, smoothness or uniformity provides the necessary coating fiim continuity. By the practice of the present invention, turbulence in a coating material and doctoring air are greatly reduced or eliminated which in turn reduces foam, allows higher coating speeds and higher coating material viscosities. In addition, the practice of this invention provides for a wide flexibility and control over coating variables such as coating viscosity, speed, air pressure, and coating thickness while maintaining a uniform coating.

Further advantages of the apparatus of the invention are its utility and simple construction. Any flexible moving web may be coated by the apparatus and method of this invention and many coating materials may be used, such as pigmented coatings, latex-based coatings and solvent-type coatings. This invention is not limited to coating a web on one side only. Multiple coatings can be laid on one or both surfaces of one web employing two or more bar means and air knives. The apparatus and method of the present invention may also be employed to impregnate or saturate a web with a desired material.

Having thus described the invention in preferred embodiments, we claim:

1. A method of applying a smooth and high gloss coating of a desired Wet thickness of a coating material onto a moving web, which method comprises (a) applying said coating material onto at least one surface of said web to produce a coated web,

(b) positioning a bar means between a point where said coating material is applied onto said web and a point P on said coated web where P is before the first point of contact of a backing means with the coated web,

(c) removing a desired amount of excess coating material by said bar means to obtain a desired coating thickness on said web,

(d) after so removing excess coating with said bar means, moving said web to one side of the bar means and thence upwardly in an arcuate course with said surface having the coating material thereon forming the outer convex side of the web where it moves in such course, and

(e) directing a stream of gas downwardly at said surface of said coated Web, at an impingement line transverse to the direction of web travel, and on the outer convex side of the web where it moves in said arcuate course, to obtain a uniform, smooth, and high gloss coating surface; said impingement line being a suflicient distance away from and beyond said bar means so that the main force of said stream of gas does not come in contact with said excess coating material being removed by said bar means.

2. The method of claim 1 wherein said gas is air.

3. A method of applying a smooth and high-gloss coating of coating material onto a moving elongated web comprising applying the coating material onto at least one surface of the web to produce a coated surface,

after the coating material has been applied, and with the surface of the web opposite its coated surface out of contact with any backing means for the web, treating the web by moving the coated surface against a physical agency that contacts the coated surface and functions by such contact to distribute the coating material on the web,

after so treating the web, moving the web laterally of such agency, and thence upwardly in an arcuate 6 course, with said coated surface forming the outer convex side of the web where it moves over such course,

backing the web by supporting the concave side of the web where it moves over said arcuate course, and

directing a stream of gas downwardly on said coated surface of the web, at an impingement line transverse to the direction of web travel, and in a region where the web is backed and the coated surface forms the convex side of the web while moving over said arcuate course, to obtain a uniform, smooth and high-gloss coating; said impingement line being a sufiicient distance away from and beyond said agency so that the main force of the gas stream does not come in contact with any coating being distributed by said agency.

4. Apparatus for applying a smooth and high-gloss coating onto a moving web comprising applicator means for applying coating material onto one surface of the web to produce a web coated over said one surface, a backing roll spaced from the applicator means, operable to guide the web as it advances from the applicator means by supporting the surface of the web opposite the webs one surface, whereby the coated surface faces away from the backing roll on traveling over the backing roll, said web advancing over the backing roll by progressing upwardly in an arcuate path over the outside of the roll, and said backing roll supporting the web with the web free of means contacting its said opposite surface in a reach extending between said applicator means and backing roll, bar means for contacting the coated surface of the web in a region along said reach, operable to remove excess coating material, and an air knife doctor for directing a stream of gas downwardly and against the coated surface of the web where the web is supported by and passes upwardly over said backing roll, said doctor being constructed and arranged to produce an impingement line transverse to the direction of web travel, with said impingement line located a sufficient distance away from and beyond the bar means so that the main force of the stream of gas coming from the doctor does not come in contact with any excess coating material removed by said bar means. 5. The apparatus of claim 4 wherein said bar means is a rod.

6. The apparatus of claim 4 wherein said bar means is a rotating rod.

'7. The apparatus of claim 4 wherein said bar means is a rod rotating in a channel support which supports the rod.

8. The apparatus of claim 4 wherein said bar means is a Wire-wound rod.

9. The apparatus of claim 4 wherein said bar means is a rotating wire-wound rod.

References Cited by the Examiner UNITED STATES PATENTS R8. 22,605 2/1945 66a et al. 118-63 X 2,135,406 114/1938 Macdonald 117 102X 2,334,102 11/1943 Kauppietal 1l8-l1()X 2,383,964 9/1945 Grupe 118 118 X 2,632,941 3/1953 Varner 118 11s X 2,711,156 6/1955 Bauling 117-102 X 2,746,878 5/1956 Rush 117 102 X 2,995,469 8/1961 Le Claire 117 102 3,063,868 11/1962 Brandsma et al 117--102 3,084,663 4/1963 Warner "118-118 RICHARD D. NEVIUS, Primary Examiner. 

1. A METHOD OF APPLYING A SMOOTH AND HIGH GLOSS COATING OF A DESIRED WET THICKNESS OF A COATING MATERIAL ONTO A MOVING WEB, WHICH METHOD COMPRISES (A) APPLYING SAID COATING MATERIAL ONTO AT LEAST ONE SURFACE OF SAID WEB TO PRODUCE A COATED WEB, (B) POSITIONING A BAR MEANS BETWEEN A POINT WHERE SAID COATING MATERIAL IS APPLIED ONTO SAID WEB AND A POINT P ON SAID COATED WEB WHERE P IS BEFORE THE FIRST POINT OF CONTACT OF A BACKING MEANS WITH THE COATED WEB. (C) REMOVING A DESIRED AMOUNT OF EXCESS COATING MATERIAL BY SAID BAR MEANS TO OBTAIN A DESIRED COATING THICKNESS ON SAID WEB, (D) AFTER SO REMOVING EXCESS COATING WITH SAID BAR MEANS, MOVING SAID WEB TO ONE SIDE OF THE BAR MEANS AND THENCE UPWARDLY IN AN ARCUATE COURSE WITH SAID SURFACE HAVING THE COATING MATERIAL THEREON FORMING THE OUTER CONVEX SIDE OF THE WEB WHERE IT MOVES IN SUCH COURSE, AND (E) DIRECTING A STREAM OF GAS DOWNWARDLY AT SAID SURFACE OF SAID COATED WEB, AT AN IMPINGEMENT LINE TRANSVERSE TO THE DIRECTION OF WEB TRAVEL, AND ON THE OUTER CONVEX SIDE OF THE WEB WHERE IT MOVES IN SAID ARCUATE COURSE, TO OBTAIN A UNIFORM, SMOOTH, AND HIGH GLOSS COATING SURFACE; SAID IMPINGEMENT LINE BEING A SUFFICIENT DISTANCE AWAY FROM AND BEYOND SAID BAR MEANS SO THAT THE MAIN FORCE OF SAID STREAM OF GAS DOES NOT COME IN CONTACT WITH SAID EXCESS COATING MATERIAL BEING REMOVED BY SAID BAR MEANS. 